Enhanced boling of water droplets containing dissolved gases or solids

摘要

We conducted experiments on the effect of dissolving either a gas (carbon dixode) or a salt (soldium carbonate or sodium bicarbonate) in water droplets bioling on a hot stinles steel surface. Substate temperatures were varied from 100 deg C to 300 deg C. We recorded the evaporation of droplets with a video system, and photographed droplet impact usingshort-duration flash photography. At surface temperatures that were too low to initiate nucleate boiling, dissolved salts were found to reduce the evaporation rate since they lower the vapor pressure of water. Dissolved gas had the opposite effect: it came out of solution and formed bubbles in the liquid, enhancing evaporation. In the nucleate boiling regime dissolved carbon dioxide enhanced heat transfer by a small amount. However, sodium carbonate prevented coalescence of vapor bubbles and produced foaming in thedroplet, greatly enhancing heat transfer and reducing the droplet, enhancing heat transfer and reducing the droplet lifetime to approximately half that of a pure water drop. Sopdium bicarbonate, which decomposes to give carbon dixide and soldium carbonate when heated, produced temperature was raised above the leidenfrost temperature of water, droplets went into film biling and bounced off the surface following impact. Dissolved carbon dixoide was found to suppress heter ogeneous bubble form ation in the droplet during impact. However, dissolved salts promoted bubble formation and led to droplet break-up during impact.